1. Field of Application
The present invention relates to a barrier discharge type of ignition apparatus for an internal combustion engine.
2. Background Technology
In recent years, internal combustion engines have become required to achieve is lower fuel costs and decreased levels of CO2 emission. To achieve this, high-efficiency engines are being developed which provide high output power but are small in size, and produce low amounts of NOx (nitrogen oxides) emissions. Such engines require techniques such as turbocharging or supercharging, higher compression ratio, higher air/fuel ratio, etc. However such techniques result in an environment within the combustion chamber which renders it difficult to reliably and quickly achieve ignition at the required ignition timings, using conventional types of ignition apparatus. There is thus a requirement for a new type of ignition apparatus which can overcome this difficulty.
One such new type of ignition apparatus is a barrier discharge (also known as dielectric-barrier discharge) ignition apparatus, having two axially extending electrodes, one circumferentially enclosed in the other, with a layer of dielectric material formed over one of the opposing surfaces of the two electrodes. The space between the electrodes is exposed to the combustion chamber of an internal combustion engine. When a short-duration high-frequency high-voltage AC burst is applied between the center electrode and outer electrode from a high-voltage AC power source, a plasma is formed in the gap between the electrodes, for igniting a fuel-air mixture within the combustion chamber and thereby igniting the mixture within the combustion chamber.
Such a barrier discharge ignition apparatus is disclosed in Japanese patent publication No. 2010-37949 (referred to in the following as reference document 1). The inventor proposes an improved barrier discharge ignition apparatus in which the discharge gap between the electrodes of the device (i.e., separation distance between one electrode and the dielectric layer formed on the opposing electrode) is varied along the longitudinal (axial) direction of the device. However it has been found by the assignees of the present invention and others based upon results from extensive testing of devices configured as described in reference document 1, that with such a configuration, the energy which is discharged in the discharge gap is not effectively utilized in effecting ignition.
Furthermore, it has been found that to achieve a high lean-limit A/F ratio (where “lean-limit A/F ratio” signifies the maximum value of air-to-fuel ratio for which stable to ignition can be achieved) it is necessary for a substantially high AC frequency to be generated by the high-voltage AC power supply. The use of a high frequency is undesirable, since only a limited amount of electrical power is available on a motor vehicle, and the electrical energy applied to effect ignition should be used as efficiently as possible. Since the energy is consumed by the ignition apparatus in producing momentary is discharges (streamer discharges) which are synchronized with the peaks of the AC voltage, the higher the frequency, the greater becomes the amount of power that must be supplied from the high-voltage AC power source. In addition, the manufacturing cost of the power source will rise in accordance with increase of the required AC frequency.
It has been found by the assignees of the present invention that these disadvantages are basically due to the fact that the tip of the center electrode does not protrude beyond the tip end of the outer (ground potential) electrode. Hence the discharge space within which the plasma is generated is separated (with respect to the axial direction) from the tip end of the outer electrode, and so is not directly exposed to the interior of the combustion chamber.
Furthermore when the size of the discharge gap varies along the axial (elongation) direction of the device, as with the type of device proposed in document 1, there is only a probability that discharge will occur at any specific gap position. In particular since the combustion chamber pressure at the ignition timing will vary, when the engine runs under various different operating conditions, it cannot be ensured that discharge will occur at any particular gap position, even if other conditions remain unchanged. Hence it becomes difficult to ensure satisfactory ignition performance.
Furthermore, with the type of device proposed in document 1, when there is a change in the (axial) position of the discharge space, due to a change to a different discharge gap, then (as can be understood from FIGS. 4 and 6 of document 1 for example), the volume of the discharge space will vary accordingly. However it is important to set an appropriate size for the discharge space. Specifically, if the volume of the discharge space exceeds a certain value (for example, 300 mm3), the electrical energy expended within the discharge space will not be used effectively to ignite the fuel/air mixture. Hence there can be a substantial waste of electrical energy.
It has further been found that with the type of ignition apparatus proposed in document 1, when discharge occurs and an initial-stage combustion flame is produced by ignition of the fuel/air mixture, the flame does not immediately propagate to the interior of the combustion chamber. This delay during which the flame remains within the discharge space may result in overheating of the dielectric material, which can cause pre-ignition.